ArrayConcat regression recover after r20312 (appeared on dromaeo benchmarks).

BUG=chromium:358561
LOG=N
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/597103003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24269 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/builtins.cc

@@ -987,12 +987,12 @@ BUILTIN(ArrayConcat) {
   Handle<FixedArrayBase> storage(result_array->elements(), isolate);
   ElementsAccessor* accessor = ElementsAccessor::ForKind(elements_kind);
   for (int i = 0; i < n_arguments; i++) {
-    // TODO(ishell): It is crucial to keep |array| as a raw pointer to avoid
-    // performance degradation. Revisit this later.
+    // It is crucial to keep |array| in a raw pointer form to avoid performance
+    // degradation.
     JSArray* array = JSArray::cast(args[i]);
     int len = Smi::cast(array->length())->value();
-    ElementsKind from_kind = array->GetElementsKind();
     if (len > 0) {
+      ElementsKind from_kind = array->GetElementsKind();
       accessor->CopyElements(array, 0, from_kind, storage, j, len);
       j += len;
     }

src/elements.h

@@ -146,9 +146,10 @@ class ElementsAccessor {
       uint32_t destination_start,
       int copy_size) = 0;
 
-  // TODO(ishell): Keeping |source_holder| parameter in a non-handlified form
-  // helps avoiding ArrayConcat() builtin performance degradation.
-  // Revisit this later.
+  // NOTE: this method violates the handlified function signature convention:
+  // raw pointer parameter |source_holder| in the function that allocates.
+  // This is done intentionally to avoid ArrayConcat() builtin performance
+  // degradation.
   virtual void CopyElements(
       JSObject* source_holder,
       uint32_t source_start,